This invention pertains to on-board automotive diagnostic systems. More specifically this invention pertains to a passive model-based diagnostic method for vehicle exhaust gas recirculation (EGR) systems.
The modern automobile uses computer executed processes to control engine and transmission operating conditions in order to maintain good fuel efficiency and reduce undesirable exhaust emissions. Associated with each engine-transmission combination is a computerized powertrain control module (PCM) that regulates powertrain functions such as fuel injection, spark advance, ignition and exhaust gas recirculation and transmission shift functions to achieve these goals. Basically, the engine and transmission are operated so as to satisfy the driving requirements of the operator while conserving fuel and reducing unwanted exhaust emissions.
The exhaust from a gasoline fueled internal combustion engine contains nitrogen, nitrogen oxides of various oxygen content (NOx), oxygen, water, carbon monoxide, carbon dioxide and unburned hydrocarbons. A catalytic converter located just downstream of the engine exhaust manifold is used to promote the oxidation of carbon monoxide, the burning of residual hydrocarbons and the chemical reduction of NOx to nitrogen. It is also the practice to recirculate suitable quantities of exhaust gas from the exhaust manifold into the air induction system during certain periods of engine operation.
Exhaust gas recirculation (EGR) systems have been an integral part of automotive engine systems for many years. They permit the diversion of controlled amounts of engine exhaust from the exhaust manifold or pipe and recirculation of the hot gas into the air induction system of the engine. Under normal engine operating conditions, engine cylinder temperatures can reach more than 3000xc2x0 F. during combustion. The higher the temperature, the more chance the engine will have NOx emissions. By introducing exhaust into the cylinders of the engine, the combustion temperature is reduced and the amount of NOx emissions is significantly reduced.
The PCM is programmed to control the timing and mass flow rate (in, e.g., grams per second) of the EGR stream. EGR is turned off during cranking, cold engine temperature (engine warm-up), idling, and acceleration or other conditions requiring high torque. There are many EGR designs or arrangements. Fundamental to all EGR designs is a passageway or port connecting the exhaust and intake manifolds. The gas pressure in the exhaust manifold is normally higher than the pressure in the intake manifold and exhaust gas can flow to the intake manifold if permitted. A valve is positioned along this passageway that regulates the recirculation stream from zero exhaust gas return flow to some maximum mass flow rate. The control system has proportional control over the EGR valve opening and thereby over the amount of EGR flow. In general, the maximum EGR flow rate is less than about twenty percent of the total flow of air and exhaust gas in the intake manifold.
United States government regulations exist to limit the amount of allowable emissions from standard passenger vehicles. One of the most stringent requirements, encompassed within California""s On-Board Diagnostics II (OBD II) regulations, require automakers to not only meet nominal emissions levels for healthy vehicles, but to also equip standard passenger vehicles with an on-board diagnostics system. This system is required to detect certain failures among a prespecified set of engine components which result in a fifty percent increase in emissions over the allowable nominal levels. Actual system performance is evaluated by connecting an emissions bag to the vehicle tailpipe and collecting and analyzing all exhaust gases generated while driving through the Federal Test Procedure (FTP) driving cycle. This procedure is conducted with both healthy vehicles and vehicles with mechanically xe2x80x9ccreatedxe2x80x9d faults.
Government regulations for EGR valve operation are focused upon detection of reduced exhaust gas flow, i.e. identification of fully or substantially blocked EGR valves. Such valves are deemed to be faulty because of their adverse effect on NOx emissions. In many current production vehicles the on-board EGR diagnostic addressing this issue relies upon an active test. The vehicle computer control module conducting the diagnostic test waits until a favorable engine operating condition is encountered. Typically the active diagnostic test is conducted with the throttle closed at a moderate to high engine speed as might be encountered during a downhill coast at a vehicle speed greater than thirty miles per hour. Once the proper operating condition is encountered and identified, the diagnostic issues an active EGR spike command. That is, a command is issued to suddenly open the EGR valve momentarily. The command is called active because it is an input to the vehicle which is neither a direct or indirect result of a specific driver request. This spike is about one second in duration and has a peak amplitude corresponding to roughly a 40 percent opening of the EGR valve.
Each modern vehicle has a mass air flow sensor (MAF) and an intake manifold absolute pressure (MAP) sensor that continually supply data to the powertrain control module. The MAF and MAP sensor measurement data for this EGR active test period illustrate the effect on these quantities. The MAF response is relatively flat during the test, corresponding to the fact that the throttle and idle valve positions were essentially unchanged during this period. By contrast, the MAP measurement features a clear spike corresponding to the active EGR spike command. Almost immediately following the end of the EGR command spike, the MAP level returns to its original value prior to the spike. Thus, the entire magnitude of the transient MAP spike is attributed to the EGR command spike, which allows a temporary infusion of exhaust gas into the intake manifold resulting in a momentarily elevated MAP.
For an unrestricted EGR valve, it is possible to analytically compute and experimentally confirm the expected magnitude of the MAP increase associated with a given EGR spike command. Automobile manufacturers have mathematical models that predict the impact of an EGR induced perturbation of MAP. In the on-board EGR diagnostic this value is compared with actual measured MAP increase. If for any reason the EGR valve flow is restricted, this actual measured MAP increase will be reduced. If the magnitude of this increase falls below a designer determined threshold, then the EGR valve operation is deemed faulty.
There are two primary drawbacks with the existing EGR diagnostic. First, since the diagnostic is xe2x80x9cactivexe2x80x9d it involves issuing a non-driver requested command to the vehicle, i.e. the EGR command spike. This results in a very slight xe2x80x9cjerkxe2x80x9d in vehicle response, which is perceptible to some drivers and negatively impacts vehicle drivability. Second, and perhaps more important, due to the timing of the EGR command spike, which is selected to optimize detection capability, running the EGR diagnostic itself generates a significant amount of emissions. Under normal driving conditions the active EGR diagnostic runs about once every vehicle trip. This diagnostic typically runs only once during the FTP cycle test but contributes a major portion (i.e. 20-25%) of the total NOx emissions collected during this test. Since these emissions are required to remain below specified nominal levels, elimination or reduction of emissions associated with this active test would be beneficial.
It is an object of this invention to provide a method of detecting a flow restriction in the EGR system without interfering with vehicle operation.
This invention provides a completely passive EGR diagnostic algorithm and an error processing method for determining whether an automotive vehicle EGR system is healthy or faulty.
The passive EGR diagnostic algorithm uses two math models to represent different states of EGR system health. In a preferred embodiment of the invention, vehicle operating conditions including mass air flow (MAF), EGR valve command, engine speed and barometric pressure are used in the models to produce estimated values of manifold absolute pressure (MAP). A first model is used to estimate a value for MAP assuming that there is no flow restriction in the EGR valve or system. If the EGR system is free of defects (i.e., it is healthy) then, ideally, the measured MAP value is close to the estimated value of MAP for a healthy EGR system. A second math model is used to estimate MAP assuming a sufficiently blocked EGR value, a fault requiring detection by the on-board diagnostic system. For example, if the normal, unobstructed flow diameter of an EGR valve is 10 mm, the model for the faulty obstructed EGR system might assume the faulty flow diameter to be 3 mm. Again, if the EGR system is healthy, the measured value of MAP should be close to the first estimate and appreciably different from the second MAP estimate. On the other hand, if the EGR system has a significant restriction (from 10 mm to 3 mm diameter) then the measured MAP should be close to the estimated value for MAP in a faulty EGR system.
Suitably, the models focus on air flow and exhaust recycle flow in the engine intake manifold and on the pressure in the manifold. One model is employed to estimate a regularly measured condition of the vehicles induction system such as MAP assuming normal unrestricted flow through the EGR valve. One or more additional models are employed to simulate the effect of a defective EGR system on MAP. Preferably one model is employed that is intended to mimic an EGR fault required for detection by an on-board diagnostic system. Again the second model estimates an already sensed or measured engine operating parameter such as MAP. Thus, a first important aspect of the invention is the use of math models to estimate contrasting operating aspects of the vehicle""s EGR system. A second important aspect of the invention is that both estimated values are compared with the measured current engine value.
In the comparison of estimated healthy and faulty MAP values, for example, with the current measured MAP value, mathematical steps are employed to reduce the effect of signal noise and other distortions and to reach a normalized comparison of the values that suitably indicates whether an EGR fault actually exists. In this approach, an EGR fault is declared if, and only if, both 1) the discrepancy between the MAP measurements and healthy MAP estimates is sufficiently large and 2) the discrepancy between the MAP measurements and a xe2x80x9cfaultyxe2x80x9d MAP estimate is sufficiently small. Preferably, the mathematical result of the comparison of estimated and measured MAP values is compared in the PCM with experimental engine data produced from representative healthy and faulty EGR systems. Thus, in accordance with the process of this invention, an EGR fault is not declared solely on the basis of a deviation from supposed normal system behavior. In addition, the resulting MAP measurement signature must adequately resemble a predicted MAP signature obtained from a faulty EGR system model.
Other objects and advantages of the invention will become more apparent from a detailed description of the specific embodiments of the invention which follow.